Devices for injecting preheated air into a shft furnace, known generally as "blast connections", involve problems of movability and sealing. In fact, as a result of the high temperature of the preheated air (a temperature of the order of 1,200.degree. C. or more) and the high temperature prevailing inside the furnace, the wall of the latter as well as the circular pipeline and the blast connection are exposed to thermal expansions and deformations which cause appreciable relative shifts between the circular pipeline and the wall of the furnace. The blast connection must therefore be capable of compensating thee relative shifts, while at the same time preventing leaks of gas or preheated air.
To meet these requirements, U.S. Pat. No. 3,766,868, the disclosure of which is incorporated herein by reference, provides a blast connection of the type described above. This blast connection has since been improved by the design of universal ball-and-socket joints of the type described in the document DE-C2-2,218,331. The three joints of this blast connection make is possible to compensate all the relative movements between the circular pipeline and the wall of the furnace. Sealing in the region of the joints is obtained by means of concertina-type compensators, while mechanical stability is ensured by means of cardan connections associated with the two opposite ends of the central tubular element in the region of the first and second ball-and-socket joints.
To reduce the amount of angular shifts of the central element and thus relieve the compensators, it is desirable that the distance between the points of articulation of this element be as great as possible. On the other hand, to reduce the bulk and cost of the blast connection, it is desirable to reduce its dimensions.